Conventionally, there have been proposed various types of buckle devices configured to prevent unintentional release of a tongue plate connected to webbing, from a buckle main body in a vehicular emergency situation.
For instance, Japanese Laid-open Patent Application Publication No. 10-71007 discloses a safety belt buckle having a housing plate with a load bearing frame secured thereto, and inside the load bearing frame is mounted a pair of right and left inertial mass bodies, each approximately semicircular-shaped in cross section, in a movable manner on a second arm of a locking lever. By depressing a release button, a catch bolt slides in parallel to release a male lug, and also the second arm is rotated toward the release button so that the pair of right and left inertial mass bodies is rotated toward the housing plate.
Further, in the safety belt buckle, when a pretensioner is activated, the buckle experiences abrupt acceleration in a direction to tighten the webbing, so that the pair of right and left inertial mass bodies abuts on a first rotation stopper which is arranged opposite thereto and shapes approximately semicircular in cross section. As a result, the rotation toward the housing plate is prevented, so that the locking lever will not rotate and the unintentional sliding of the release button can also be prevented.